chapter 10- bio unit 4

fossil record

 the information derived from fossils. The fossil record is arranged in chronological order and helps us map the history of life on Earth, placing species in the appropriate geologic time frame

general trajectory of the Earth’s evolutionary history

• all living things on Earth evolved from a single celled prokaryote

• evolved the ability to photosynthesis. this caused the rise of simple eukaryotes

• multicellularity evolved, followed by the cambrian explosion when most major animal groups first appeared

fossil

the preserved body, impressions, or traces of a dead organism

fossilisation

 the process by which an organism becomes a fossil

fossilisation steps

1. Cover: Remnants of an organism are rapidly covered by sediment, meaning that the dead organism is not exposed to oxygen, microorganisms, and other disturbances that would increase its rate of decomposition.

2. Sedimentary rocks: Over time, sediment layers build upon each other and compact, layer by layer, until pressure cements them together to form sedimentary rock.

3. Fossilisation: Within this rock, the fossilised remains can take many forms, including a permineralised, mould, or cast fossil.

permineralised fossil

 fossil formed when mineral-rich groundwater deposits minerals like silica and calcite into organic material, creating a mineral relic

mould fossil

fossil formed when a living thing decomposes underneath sediment, creating a cavity in the shape of the dead organism

cast fossil

 fossil formed when a mould fossil is filled with sediment

trace fossil

fossil or structure indicating the presence of organisms, rather than the organisms themselves (e.g. nests, footprints, and burrows)

conditions that increase the likelihood of fossilisation

• physical protection from scavengers and decomposers (e.g. fungi, bacteria)

• areas of rapid sediment accumulation

• constant cool temperatures

• low oxygen availability

• low light exposure.

relative dating

a dating technique used to determine the relative age of a fossil by comparing its position to other fossils or rock in surrounding rock strata (layers)

law of fossil succession

Older fossils are found in lower layers of strata, younger fossils are found in upper layers of strata. Fossils increase in complexity from the lower layers to the top layers. And the layers of strata are in chronological order

geological timescales

The law of fossil succession also means that we don’t necessarily need to have another fossil to compare against in order to assign our new fossil a relative age. Instead, we only need to be able to determine the age of the rock stratum in which the fossil is found.

index fossils

 a group of widespread fossils which existed for a short period and have a known age. Can be used as a reference to easily determine the age of unknown fossils

transitional fossils

fossilized remains that exhibit characteristics of both an ancestral group and its derived descendant group, showcasing intermediate evolutionary stages

absolute dating

absolute dating techniques can be used to calculate the absolute age of a fossil in years which is the estimate of the age (in years) of a fossil or rock.

Radioactive dating is one of the most commonly used methods for determining the age of fossils and involves comparing the ratio of radioactive isotopes (radioisotopes) found inside the fossil to the relatively stable amount found in the atmosphere.

radioactive isotope

 a radioactive atom of a specific element. This atom breaks down into a predictable and stable product. Also known as a radioisotope

absolute dating 3 principles (U, C, H)

1. Unstable elements: Radioisotopes are unstable elements that will break down over time into a more stable product. For instance, carbon-14 (a radioisotope) will break down into nitrogen-14.

2. Constant rate of breakdown: While these radioisotopes can break down at any point, on average the rate of breakdown is constant and can be modelled. One of the ways in which we model this breakdown is by calculating the half-life of that radioisotope.

3. Half life: Half-life describes the amount of time before half of the mass of a radioisotope is broken down into predictable and stable products. For example, carbon-14 is a radioisotope that has a half-life of 5 730 years. This means that after 5 730 years of an organism’s death, half of its carbon-14 atoms will have broken down into nitrogen-14 atoms

principles of radiocarbon dating

1. All living things contain carbon. This carbon exists as a ratio of two isotopes – 12C (a stable isotope) and 14C (a radioactive isotope). The ratio of these two isotopes will be the same as the ratio in the atmosphere, given that carbon is constantly being cycled between the organism and its environment while it is alive.

2. 14C begins to decay: When the organism dies, its 14C will begin to decay. This is because 14C is radioactive and breaks down into 14N (a stable isotope). While this decay occurs, the carbon in the dead organism will not be replaced by existing carbon in the atmosphere causing the ratio between the two isotopes to change.

3. Ratio to measure: At any point, scientists can measure the amount of 14C present in the fossil and determine how long ago it died. This is done by comparing the 14C : 12C ratio in the fossil to the ratio of 14C : 12C in the atmosphere.

homologous structures

• features present in two or more species that may look and function very differently in each species, but are derived from a common ancestor

• physical evidence of divergent evolution

• This process typically occurs as a result of individual populations adapting to different selection pressures or genetic drift which alters population genomes over extended periods of time.

analogous structures

• features present in two or more species that fulfil the same function but do not originate from a common ancestor

• this is evidence of convergent evolution

divergent evolution

n the process in which a common ancestor evolves into two or more descendant species

convergent evolution

 the process in which distantly related species evolve similar traits over time due to the action of similar selection pressures

vestigial structures

 features that have lost all or most of their usefulness as a result of evolution by natural selection

Despite having no function, these structures often remain in a species as they are not selected against

amino acid sequence similarity

Comparing amino acid sequences is another means of determining how related different organisms are

molecular homology

• the study of the similarities in the nucleotide sequences of DNA or amino acid sequences in proteins between organisms to establish relatedness

• we analyse proteins from conserved genes whcih are genes that have remained largely unchanged throughout evolution

DNA sequence similarities

• DNA sequences of different species can be compared to determine the evolutionary relatedness between them.

• a higher similarity in DNA sequence implies a closer level of relatedness between different organisms.

• We can compare the differences in the DNA sequences of different species simply by looking at the order of these bases at corresponding gene regions

analysing amino acid sequences vs DNA sequences

• both help determine the relatedness between different organisms

• depends on how related the species are

• a limitation to analysing amino acid sequences is that closely related species are likely to have very similar amino acid sequences

• in these instances, scientists determine relatedness by comparing nucleotide sequences, looking for silent mutations

phylogenetic trees

 a diagram used to show the relatedness between organisms

phylogenetic trees are useful in displaying

• the timeline of lineages

• relatedness between taxa

• shared characteristics of different taxa.

root

A line at the origin, representing the earliest common ancestor

branch

Each line on the phylogenetic tree

node

A point where the branches split from each other, representing a divergence between those two taxa

leaf

The end of a branch, representing where the present-day or extinct species are found. They are labelled with the species or taxa name

3 characteristics of index fossils and how they are used to date other fossils

Index fossils are abundant, distinctive and found in a wide geographical area. Index fossils are used for relative dating. Fossils above the index fossil are younger. Fossils below the index fossil are older

2 ways sympatric speciation